Sustainable development most prominently entered the global political arena in 1987 in a report from the United Nations Commission on Environment and Development, also known as the Brundtland report. In response to the concept of sustainable development, a vast array of ideas, concepts, methods and tools to aid organizations and governments in addressing the socio-ecological problems has been developed. Though helpful in many contexts, the multitude of such support also risks creating confusion, not the least since there is no generally endorsed overriding and operational definition of sustainability. Thus, there is a growing need for such a definition and for an understanding of how these ideas, concepts, methods and tools relate to sustainability and to each other. A framework for strategic sustainable development (FSSD) has been developed over the last 20 years to create such a unifying structure. The aim of this research is to contribute specifically to the social sustainability definition of this framework. The research follows the Design Research Methodology. First, the social dimension of the FSSD as it stands currently was examined and described as was the general field of social sustainability. Then, a new approach to the social side of the FSSD was created. The studies revealed that the field of social sustainability, in general, is vastly under-theorized and under-developed, and that a clear framework is important and desired. They also laid out in which ways specifically the structure of the FSSD could be used to further develop the social dimension of strategic planning and innovation, and that currently this aspect of the FSSD is relatively under-developed. This assessment was followed by a first attempt at a clearer definition of social sustainability. Based on these explorations, this research suggests five principles as a hypothesis to be used as a definition of social sustainability, the key-terms of which being ’integrity’, ‘influence’, ‘competence’, ‘impartiality’ and ‘meaning’. For validity purposes the results were cross-checked with other approaches and theories. The validity check shows that similar key-terms have been found by other researchers. In conclusion, this research contributes with a hypothesis for a clearer definition of social sustainability, which is general enough to be applied irrespective of spatial and temporal constraints, but concrete enough to guide decision-making. This is a contribution to systems science in the sustainability field and it is a step to creating an enhanced support for strategic planning and innovation for sustainability. Further testing and refinement of this theoretical foundation, and bringing it into practical use, will be the subject of the continued studies.